1. Field of the Invention
The invention relates to an optical system of optical pickup for use in an optical recording and reproducing apparatus that records and reproduces information to and from optical discs having different thicknesses of the substrate or designed for different wavelengths. More particularly, the invention relates to an optical pickup device.
2. Description of Related Art
As a type of optical recording and reproducing apparatuses, there is an optical disc player which can read recording information from an optical disc such as the so called LD (laser disc), CD (compact disc), DVD (digital versatile disc, or digital video disc), or the like used an optical recording medium. There also is a compatible disc player which can read information from any one of those optical discs.
Among those optical discs as optical information recording media, a numerical aperture NA of an objective lens, a thickness of substrate, an optimum reading light wavelength, and the like are designed by different specifications. To realize an optical pickup of a CD/DVD compatible player, therefore, it is necessary to compensate at least two differences of the numerical aperture NA and the substrate thickness.
An objective lens which is used in a general optical pickup using a semiconductor laser is manufactured in consideration of the substrate thickness of the optical disc and its corresponding wavelength. As for an optical disc having a different thickness or corresponding wavelength, since a spherical aberration occurs, the recording and reproduction cannot be performed. Hitherto, a substrate thickness of the optical disc which is used as an LD, CD, CD-R (CD Recordable), etc. is equal to 1.2 mm, a corresponding wavelength lies within a range of 780 to 830 nm, and a numerical aperture of the objective lens is equal to about 0.45. A substrate thickness of the DVD is equal to 0.6 mm, a corresponding wavelength lies within a range from 635 to 655 nm, and a numerical aperture of the objective lens is equal to about 0.6.
First, by increasing the numerical aperture of the objective lens, optical resolution is improved and a recordable or reproducible frequency band can be widened. If there is a tilt of the optical disc, a coma-aberration of a light spot increases to a value that is equal to or larger than the conventional one. By making the substrate thickness of the optical disc thin, however, the coma-aberration occurring due to the tilt of the optical disc can be suppressed to a level similar to the conventional one.
By reducing the use wavelength, optical resolution is improved and the recordable or reproducible frequency band can be widened in a manner similar to the case of increasing the numerical aperture of the objective lens.
However, if a CD-R having a wavelength of 780 nm (nanometers) which is used for an LD, CD, or the like is reproduced by a semiconductor laser of a short wavelength, for example, 650 nm, there will be a problem that an enough reproduction signal or control signal cannot be obtained due to such a cause as a difference of reflectance, or absorption factor of the recording surface. With the construction of the conventional compatible player as mentioned above, a reading spot is formed by a common single light source and, usually, a light source to emit a reading light having a wavelength 650 nm that is optimum to reproduce the DVD is used in common upon reproduction of a CD. It is, therefore, impossible to reproduce a recordable disc of the CD-R which can be written once by a light source having a wavelength of 780 nm.
It is thus impossible to reproduce both of a DVD corresponding to the wavelength 650 nm and the CD-R by one optical pickup. It is, therefore, necessary to provide an optical pickup using 650 nm for reproduction of a high density optical disc and an optical pickup for reproduction of a CD and a CD-R, respectively.
However, if the optical system including a prism, a lens, and the like is produced by using a plurality of light sources, there arises a problem that the optical pickup or the whole head becomes complicated and increases in size.
The invention is made in consideration of the above problems and it is an object of the invention to provide a small optical pickup apparatus of low cost in which optical discs having different thicknesses of substrates or different corresponding wavelengths can be recorded and reproduced by one optical pickup device.
According to the invention, there is provided an optical pickup device comprising:
a main portion having a first light source to emit a first laser beam, a second light source to emit a second laser beam having a wavelength different from a wavelength of the first laser beam, and a photodetecting portion to receive first and second return light of the first and the second laser beams which are reflected and returned from an optical disc; and
an optical axis coupling prism which is fixed to the main portion and has a first plane of incidence where the first laser beam enters, a second plane of incidence where the second laser beam enters, a plane of synthesis to make optical axes of the first and the second laser beams coincident, and a plane of emission to emit the first and the second laser beams which have passed through the plane of synthesis and whose optical axes are made coincident.
In the optical pickup device of the invention, the optical axis coupling prism has a reflecting surface to reflect the second laser beam to the plane of synthesis.
In the optical pickup device of the invention, the plane of synthesis is a dichroic mirror having an incident angle dependency.
In the optical pickup device of the invention, the reflecting surface and the plane of synthesis are arranged in parallel.
In the optical pickup device of the invention, the plane of emission and the first plane of incidence are arranged in parallel.
In the optical pickup device of the invention, the first and the second planes of incidence are arranged in parallel.
In the optical pickup device of the invention, the optical axis coupling prism has a first light separating device, arranged between the second plane of incidence and the plane of synthesis, for generating two sub laser beams which are deviated from the optical axis of the second laser beam.
In the optical pickup device of the invention, the plane of emission has a second light separating device for receiving the first and the second return light and dividing and converging the first and the second return light to different positions on the photodetecting portion, respectively.
In the optical pickup device of the invention, the second light separating device divides each of the first and the second return light into four light beams.
In the optical pickup device of the invention, the photodetecting portion has: a first photodetecting portion for receiving the first return light and outputting a detection signal to form a tracking error signal of a differential phase detection method; and a second photodetecting portion for receiving the second return light and outputting a detection signal to form a tracking error signal of a 3-beam system.
In the optical pickup device of the invention, the first photodetecting portion of the photodetecting portion has a region extended in the direction corresponding to the radial direction of the optical disc in order to receive both of the first and the second return light.
According to the invention, the light source is provided for every corresponding disc and the optical axes of the first and the second laser beams can be made coincident in the optical axis coupling prism, and further, a grating function and a hologram function can be added, so that the number of parts can be reduced and it is possible to easily manufacture the device. Thus, the optical pickup optical system can be simplified.